Fluid streams such as water or effluents often include contaminants such as dissolved halogenated or organic compounds, nitrogen oxides, halocarbons, pesticides, organic dyes, etc. Photocatalysts can be used to purify fluid streams by converting these contaminants into less harmful materials or substances, which may be more easily removed therefrom.
The conversion of contaminants occurs when the effluent or toxic pollutant is brought in contact with the photocatalyst illuminated by a nearly ultra-violet or fluorescent light source. The photocatalyst is either in the coated form or in dispersed form. Typically, photocatalyst is deposited on the surface of a support structure to provide a stable photo catalytic surface and to ensure that the flowing stream does not carry it away. To be effective, the contaminants must be brought into contact with the photocatalyst.
The photocatalytic oxidation reaction with TiO2 either in coated or dispersed form has been known for some time. A synthetic and commercially available TiO2 containing greater than 75 per cent of anatase can be used as a photocatalyst to degrade various organic species to render it environmentally safe. These TiO2 either coated on support material or dispersed in aqueous solution have been found to have good photocatalytic degradation efficiencies when it is desired to impact oxidation or reduction properties to organic moieties. Titanium dioxide have specific properties for its use as a photocatalyst such as (i) oxidation of water-bound environmental contaminants, irradiated with solar or simulated light; (ii) complete photo degradation of halocarbons, viz. dibromo-ethane, trichloroethylene; chloro-benzenes etc widely used as solvents in pesticides, insecticides, herbicides; (iii) photo destruction of different classes of organic dyes and biological stains from waste water; (iv) oxidation of cyanide found in rinse water of steel industry, electroplating, gold extraction, extraction in mines, to less toxic oxidation products like OCN. The latter is further oxidized to NO3− and CO32−; (v) reduction of metal ions to metallic state to remove toxic and noble metal ions from waste water; (vi) photo destruction of anionic, cationic and non-ionic surfactants under solar or simulated light using aqueous TiO2 suspensions.
The wide spectrum of surface properties such as crystallinity, high percentage of anatase, surface charge state, surface hydration/hydroxylation, combined with fine particle size and high surface area, low density and chemical inertness make titanium dioxide a potential industrial photocatalyst in the field of photo/environmental chemistry. Such a TiO2 in its porous state can be employed for the photo catalytic oxidation reaction employing simple method described hereinto.
The inorganic molecules and ions, viz. CO2, SO2, NO3, NH4+ are the reaction products of photocatalytic oxidation reaction on TiO2 surfaces. Under favourable conditions, the organic species present in (i) waste water effluents (ii) halo carbons (iii) dyes and dye stuff (iv) surfactants (v) toxic pollutants etc. undergoes photocatalytic oxidation reaction, when the TiO2 surface is irradiated with ultra violet or fluorescent tube-light.
U.S. Pat. No. 5,035,784 (1991) to M. A. Anderson et al. has disclosed the preparation of a highly porous titanium ceramic membrane and which have the propensity to absorb organic molecules, and also to degrade the complex organic molecules under UV light. The preparation involves hydrolysis of titanium alkoxide at room temperature in organic alcohol. The addition of large amount of water will precipitate titanium hydroxide, which is then peptized with HNO3 at room temperature. The suspension is heated with stirring at 85° C. and maintained it for 12 hours, whereby the colloid gel is solidified onto a support which on firing at 500° C. results in a highly porous, continuous web of sintered particles forming a rigid membrane. The drawback associated with this processes are (i) tight control of pH of the colloidal mixture; ii) any alcohol as solvent will not be adequate. The alcohol solvent is preferably an alkyl alcohol different from alkyl radical in titanium alkoxide and (iii) the firing temperature is critical as it may cause as it may cause cracking into the resulting ceramic.
U.S. Pat. No. 5,874,701 (1999) to T. Watanabe at al. discloses a process for photo catalytically treating hospital room or living space contaminated by bacteria or an interior space bearing airborne malodorous substances. It comprises of a thin film of TiO2 coated on the inner walls of the room, which is irradiated by a fluorescent lamp and photo excited by small amount of UV radiation included in the light of the lamp. The wattage of fluorescent lamp, distance between the thin film and lamp, intensity of UV light were studied to photo decompose the bacteria and chemical compounds deposited on the photo excited thin film. This process has the limitation that it can photo decompose bacteria and hazardous chemical compounds (airborne) substances. It does not claim anything about auto cleaning of stains caused by spices on kitchen walls and platforms.
U.S. Pat. No. 5,779,912 (1998) to A Gonzalez Martin et al. discloses a method and apparatus for mineralizing organic contaminants in water or air by photocatalytic oxidation in a unique two-phase or three-phase boundary system in a photocatalytic reactor, which works effectively at ambient temperature and low pressure. The semi-conductor TiO2 is coated by different technique on porous substances viz. porous polymers; porous metal; porous carbon or graphite; or porous ceramic in order to have passage there through for different oxidant, used in different proportion in the decomposition process. The drawback associated with this process is that the system is effective when binary metal oxides are selected as photocatalyst, and at low pressure. The presence of oxidant is quintessential for photodecomposition.
Japanese Patent No 9,276,694 (1997) to O. Taware discloses a method which involves the preparation of a paste from TiO2, glass powder and water and applying a thin film of this paste on ceramic surface prior to calcination, in order to obtain a stiff and less porous TiO2 layer that has strong adherence to substrate and long service life. The drawback associated with method is that this photocatalyst is compatible for decomposition of nitrogen and phosphorous compounds, for cleaning air, exhaust gas and water.